The present invention relates generally to programmable sweep generators, and more particularly to a sweep generator for generating a sweep signal, the slope or sweep rate of which may be switched in predetermined steps.
Sweep (or ramp) generators are widely used in electronic instrumentation, one example of which is in an oscilloscope, wherein accurate sweep signals are generated in predetermined steps to provide the timebase of a quantitative waveform measurement. Sweep generators for typical oscilloscopes generate timebase sweep signals ranging from ten nanoseconds per screen division to five seconds per screen division in 1-2-5 steps.
Shown in FIG. 1 is a conventional sweep generator which comprises a controllable current source including transistor Q.sub.1, a plurality of timing resistors R.sub.1, R.sub.2, . . . , R.sub.n, switches S.sub.21, S.sub.22, . . . , S.sub.2n, an operational amplifier OP, a plurality of timing capacitors C.sub.1, . . . , C.sub.m, switched by respective switches S.sub.1, . . . , S.sub.1m, a gating transistor Q.sub.2 and a buffer amplifier A. A reference voltage V.sub.ref is applied to the non-inverting input of operational amplifier OP to maintain the emitter potential of transistor Q.sub.1 equal to V.sub.ref regardless of the output current from the controllable current source, the collector current of transistor Q.sub.1, therefore depending only on the selected timing resistor R.sub.1, R.sub.2, . . . , or R.sub.n. Such output current is commonly referred to as a timing current and is utilized to charge the selected timing capacitor C.sub.1, . . . , or C.sub.m while gating transistor Q.sub.2 remains non-conducting, thereby developing a linear ramp voltage across the timing capacitor to be made available via buffer amplifier A to output terminal OUT. The sweep rate is proportional to the timing current but inversely proportional to the capacitance of timing capacitor. Gating transistor Q.sub.2 is turned on by applying a relatively positive voltage to the base thereof to discharge the timing capacitors C.sub.1, . . . , C.sub.m, thereby forming a short retrace portion of the ramp signal. Buffer amplifier A may include a source follower field effect transistor (FET) input stage and an emitter follower output stage for impedance conversion, thereby minimizing changes in the timing current due to leakage (or bias current), which may cause timing error.
Disadvantages of the conventional sweep generator include the need for many precision timing elements (capacitors and resistors) including timing resistors in the order of ten kilohms to over ten megohms and difficulty in implementing electronic switches. For example, any resistance of the switches S, in series with the timing capacitor would develop an undesirable voltage step when the timing current is supplied thereto. Such a voltage step is not negligible as the timing current becomes larger.